Tried basic remedies: Creating default object from empty value; /wp-admin/includ

I run multiple WP sites on the same server. Only one is having a problem.

Warning: Creating default object from empty value in …blah…/wp-admin/includes/post.php on line 567

I’ve already tried the following (thank you for everyone who has suggested these; I’ve tried them multiple times)

No plugins enabled
Theme 2012
Reinstall WP core files
Re-upload WP core files
Rename plugin folder
georgestephanis/object-cache-flusher-button

Version of PHP is the same (5.4.34) on all my sites, only one site has a problem.

Other clues:

Also cannot make a new page or upload media (it uploads but stops at crunching)

On new post, there is no save button, only submit for review

Line 567 of /wp-admin/includes/post.php is:
$post->post_content = apply_filters( ‘default_content’, $post_content, $post );

Another clue: I disabled all plugins to troubleshoot. When re-enabling jetpack it won’t reconnect to wordpress.com (again this problem with only one of my many sites) saying missing_secrets

My only hunch is something corrupted in the database but I don’t know if that is consistent with what line 567 is trying to do.